Photon is electromagnetic field, right?


by Barry_G
Tags: electromagnetic, field, photon
DaleSpam
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#37
Dec12-12, 10:44 AM
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Quote Quote by Barry_G View Post
Maxwell's equations do not explain how can photon have electric and magnetic field and yet we can not bend a beam of light by external electric or magnetic fields.
Yes, they do. Specifically, the linearity of Maxwell's equations shows this, as mentioned above.
Barry_G
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Dec12-12, 10:45 AM
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Quote Quote by elfmotat View Post
Your responses are quite painful to read. The "photon" in question consists of an electric and a magnetic field, which he denoted as [itex]E_{\gamma}[/itex] and [itex]B_{\gamma}[/itex]. Maxwell's Equations aren't just for the narrow range of applications you list above. They completely describe how electric and magnetic fields behave!
They do not describe photons (em waves).

http://en.wikipedia.org/wiki/Maxwell...ectromagnetism : any phenomenon involving individual photons, such as... would be difficult or impossible to explain if Maxwell's equations were exactly true, as Maxwell's equations do not involve photons.


Electromagnetic wave equation is only DERIVED from them, and it is not to describe any properties of photons, but only to get to the speed of light. It also has nothing to do whether photons are neutrally charger or not, it is about working out that equation in a setup of vacuum and charge-free SPACE.
DaleSpam
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Dec12-12, 10:50 AM
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Quote Quote by Barry_G View Post
They do not describe photons (em waves).
As I said back in post #3 none of your questions are actually about photons. They are about classical EM waves. Specifically, you want to know how you can have EM fields in the absence of a charge. Maxwells equations in vacuum (and their associated wave solutions) are the answer to that question.
elfmotat
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Dec12-12, 10:59 AM
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Quote Quote by Barry_G View Post
They do not describe photons (em waves).

http://en.wikipedia.org/wiki/Maxwell...ectromagnetism : any phenomenon involving individual photons, such as... would be difficult or impossible to explain if Maxwell's equations were exactly true, as Maxwell's equations do not involve photons.
That article is talking about the failure of classical electrodynamics and why quantum electrodynamics is needed. Maxwell's Equations completely describe EM waves in the classical sense. Of course they don't involve photons as such, because a photon is not a classical concept. Notice how I put quotes around the word "photons" because what we were talking about wasn't photons exactly, it was EM waves. You're the one who keeps using the two interchangeably.

You're mixing classical and quantum ideas. Classical and quantum electrodynamics are two completely different frameworks. For the majority of this thread we've been discussing light in terms of classical EM waves. Now you're claiming I'm wrong because you suddenly decided to switch over to quantum. I'm not wrong, you're just inconsistent.

Quote Quote by Barry_G View Post
Electromagnetic wave equation is only DERIVED from them, and it is not to describe any properties of photons, but only to get to the speed of light.
You just said that Maxwell's equations don't describe EM waves, and now you're saying EM waves are derived from Maxwell's equations. Obviously if EM waves are derived from Maxwell's equations then Maxwell's equations describe EM waves.

Quote Quote by Barry_G View Post
It also has nothing to do whether photons are neutrally charger or not, it is about working out that equation in a setup of vacuum and charge-free SPACE.
You derive the equations for light in charge-free space, so obviously light is charge-free. Is this really so hard to understand?
Barry_G
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Dec12-12, 11:17 AM
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Quote Quote by DaleSpam View Post
As I said back in post #3 none of your questions are actually about photons. They are about classical EM waves.
You mean "photon" is concept that belongs to QM and has nothing to do with EM fields? Ok, yes, so I shall call it EM waves instead of photons.


Specifically, you want to know how you can have EM fields in the absence of a charge. Maxwells equations in vacuum (and their associated wave solutions) are the answer to that question.
Maxwell equations are about electric currents in wires, nothing to do with any EM waves, only electromagnetic wave equation has anything to do with light, and it is not about having EM fields in the absence of any charge, it's simply about EM fields propagating thorough empty space, but it says nothing about how would those EM field be influenced or not if there were any external fields in that space they propagate through.


Maxwell's equations. The key property of Maxwell's equations that lead to this is the linearity. The linearity of Maxwell's equations shows that EM follows the principle of superposition which in turn implies that an EM field won't be altered by passing through an external static electric or magnetic field.
There is no any superposition if you have a single wave or a single electric/magnetic field. For superposition to neutralize that field you would need another field of opposite sign.
Barry_G
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Dec12-12, 11:29 AM
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Quote Quote by elfmotat View Post
That article is talking about the failure of classical electrodynamics and why quantum electrodynamics is needed. Maxwell's Equations completely describe EM waves in the classical sense.
Maxwell's Equations do not describe any waves. Not even electromagnetic wave equation describes any waves, it's just gets you the speed of light.


You just said that Maxwell's equations don't describe EM waves, and now you're saying EM waves are derived from Maxwell's equations. Obviously if EM waves are derived from Maxwell's equations then Maxwell's equations describe EM waves.
Maxwell equations are about electric currents and wires, nothing to do with any waves.


You derive the equations for light in charge-free space, so obviously light is charge-free. Is this really so hard to understand?
No, it means SPACE is charge free. It means there are no any external fields that could influence those fields making up em wave. Is that so hard to understand?
Dead Boss
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Dec12-12, 11:30 AM
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It seems quite obvious now that Barry does not want to learn anything and is trying to push his own personal theories. Is there a way to request thread lock?
elfmotat
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Dec12-12, 11:31 AM
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Quote Quote by Barry_G View Post
Maxwell equations are about electric currents in wires,
NO NO NO NO NO!!! Maxwell's Equations tell us how electric and magnetic fields behave in any situation, with any charge and current distribution! They're not just about "electric currents in wires!"


Quote Quote by Barry_G View Post
nothing to do with any EM waves,
NO!!! As you've been told and shown numerous times already, EM waves are derived from and described by Maxwell's Equations!

Quote Quote by Barry_G View Post
only electromagnetic wave equation has anything to do with light, and it is not about having EM fields in the absence of any charge, it's simply about EM fields propagating thorough empty space, but it says nothing about how would those EM field be influenced or not if there were any external fields in that space they propagate through.
K^2 already showed you with a detailed post what happens in the presence of external fields!
elfmotat
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Dec12-12, 11:34 AM
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Quote Quote by Dead Boss View Post
It seems quite obvious now that Barry does not want to learn anything and is trying to push his own personal theories. Is there a way to request thread lock?
Agreed, he doesn't want to learn at all. He seems to enjoy arguing about things he doesn't understand just for the sake of it.
sophiecentaur
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Dec12-12, 11:41 AM
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Quote Quote by Barry_G View Post
You mean "photon" is concept that belongs to QM and has nothing to do with EM fields? Ok, yes, so I shall call it EM waves instead of photons.

Maxwell equations are about electric currents in wires, nothing to do with any EM waves, only electromagnetic wave equation has anything to do with light, and it is not about having EM fields in the absence of any charge, it's simply about EM fields propagating thorough empty space, but it says nothing about how would those EM field be influenced or not if there were any external fields in that space they propagate through.

There is no any superposition if you have a single wave or a single electric/magnetic field. For superposition to neutralize that field you would need another field of opposite sign.
I have just been wading through this thread and it strikes me that you are determined to approach the understanding of this topic entirely on your own ideosyncratic terms. You keep wanting to bend what you are told to fit your particular model. Of course you are free to believe anything you want to but, as this thread has demonstrated, you just won't get anywhere near the accepted understanding of EM waves if you don't follow the established approach.
You need to ask yourself whether you really believe you are right and that all the replies you've been given are flawed. Could you not consider starting at the very beginning and work towards some real sense instead of jumping in half way through, getting many things the wrong way round and then demanding to be given answers that make sense to you. This is a difficult topic and needs some Rigour if you want an understanding of it. I really don't think that you can accept (or even recognise) correct answers when you see them.
You need to learn the basic terms and definitions in full and not use your own interpretation of things.

(The title of the thread is slightly bonkers, by the way)
Barry_G
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Dec12-12, 11:55 AM
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Quote Quote by Dead Boss View Post
It seems quite obvious now that Barry does not want to learn anything and is trying to push his own personal theories. Is there a way to request thread lock?
Quote Quote by sophiecentaur
I have just been wading through this thread and it strikes me that you are determined to approach the understanding of this topic entirely on your own ideosyncratic terms. You keep wanting to bend what you are told to fit your particular model. Of course you are free to believe anything you want to but, as this thread has demonstrated, you just won't get anywhere near the accepted understanding of EM waves if you don't follow the established approach.
You need to ask yourself whether you really believe you are right and that all the replies you've been given are flawed. Could you not consider starting at the very beginning and work towards some real sense instead of jumping in half way through, getting many things the wrong way round and then demanding to be given answers that make sense to you. This is a difficult topic and needs some Rigour if you want an understanding of it. I really don't think that you can accept (or even recognise) correct answers when you see them.
You need to learn the basic terms and definitions in full and not use your own interpretation of things.
http://en.wikipedia.org/wiki/Maxwell%27s_equations

It's not about light or EM waves. It is about geometry of fields, about currents and wires. Do you understand?
sophiecentaur
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Dec12-12, 12:00 PM
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Quote Quote by Barry_G View Post
http://en.wikipedia.org/wiki/Maxwell%27s_equations

It's not about light or EM waves. It is about geometry of fields, about currents and wires. Do you understand?
Ummm.
Do YOU understand?
Barry_G
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Dec12-12, 12:04 PM
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Quote Quote by elfmotat View Post
NO NO NO NO NO!!! Maxwell's Equations tell us how electric and magnetic fields behave in any situation, with any charge and current distribution! They're not just about "electric currents in wires!"
Educate yourself. If you want to work out electric and magnetic fields for any distribution you need to use equations for point charges. That is Coulomb's law, Biot-Savart law and Lorentz force equations, for point charges, and then you integrate.

NO!!! As you've been told and shown numerous times already, EM waves are derived from and described by Maxwell's Equations!
Not even electromagnetic wave equation describes any waves.


K^2 already showed you with a detailed post what happens in the presence of external fields!
Nonsense.
Nugatory
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Dec12-12, 12:07 PM
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Quote Quote by Barry_G View Post
Maxwell's Equations do not describe any waves. Not even electromagnetic wave equation describes any waves, it's just gets you the speed of light.
Ahhh.... Electromagnetic waves propagating through empty and charge-free space are described by Maxwell's equations.

In fact, electromagnetic waves were predicted by Maxwell's equation before they were ever discovered. The discovery that light was a specific example of the electromagnetic waves predicted by Maxwell's equations came later.
elfmotat
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Dec12-12, 12:34 PM
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Quote Quote by Barry_G View Post
Educate yourself. If you want to work out electric and magnetic fields for any distribution you need to use equations for point charges. That is Coulomb's law, Biot-Savart law and Lorentz force equations, for point charges, and then you integrate.
Coulomb's Law is derived from Maxwell's Equations (Gauss' Law for Electricity) under the assumption of static conditions (i.e. when [itex]\partial E / \partial t =0[/itex] and [itex]\partial B / \partial t =0[/itex]).

The Biot-Savart Law is derived from Maxwell's Equations (Ampere's Law and Gauss' Law for Magnetism) also under the assumption of static conditions.

Coulomb's Law and the Biot-Savart Law are only valid when the electric and magnetic fields are not changing in time. Maxwell's Equations are more fundamental, and will describe the electric and magnetic fields under any conditions. The Lorentz Force Law will then tell you how a test particle placed in these fields will behave.

Quote Quote by Barry_G View Post
Not even electromagnetic wave equation describes any waves.
I'm not really sure what you mean by that. It's a wave equation, so obviously it describes a wave.

Quote Quote by Barry_G View Post
Nonsense.
Just because you say something is nonsense doesn't make it so. He showed you, step by step, how the linearity of Maxwell's Equations prove that external fields superimpose on the EM wave so that the wave itself is unaffected by the external fields.
jtbell
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Dec12-12, 12:46 PM
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Quote Quote by Dead Boss View Post
It seems quite obvious now that Barry does not want to learn anything and is trying to push his own personal theories. Is there a way to request thread lock?
The normal method is to use the "Report" button on a problematic post. In this case, however, you can consider it done.


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